Method and device for stacking substrates which are to be joined by bonding

ABSTRACT

A method and an apparatus are proposed in order to bond multilayered substrate packages over a large surface at uniform heating in the course of one single work step, wherein the structured substrates are to begin with aligned to one another with high precision to correspond to the structuring in a working area separate from the heating apparatus.

The subject of the invention is used in particular when fabricatingsensors built up in laminar construction. Thus for instance pressure- oracceleration-sensors can be manufactured in a particularly advantageousmanner by means of stacked substrates, wherein the substrates orsubstrata have previously been processed in a suitable manner, meaningthey have been provided with specific delicate structures.

In order to now connect the substrates to one another, varioustechnologies have already been developed. At this time we will onlymention the following methods known to the specialized community withoutany more precise explanations:

anodic bonding

silicon fusion bonding

eutectic bonding

low temperature glass bonding

All these bonding methods require that the surface of the substratesmust be polished and thoroughly cleaned, that the substrates to bebonded to one another therefore must be manipulated in as particle-freean environment as possible, that they must meet very tightspecifications as far as the evenness or flatness of the contact facesof the individual substrates is concerned and that the interconnectionof the substrates must occur so that they remain free of mechanicalstresses, in order to prevent void-like free spaces in between thesubstrates. The non-observance of these basic requirements considerablyreduces the quality of the surfaces to be joined to one another. It caneven put into question the intended function of the product to befabricated.

The present invention will be described in the following for betterunderstanding by way of an example of the method of anodic bonding.Although it is naturally not limited to this specific method, it canrather be used with other bonding methods.

Anodic bonding serves for establishment of a hermetic and mechanicallysolid connection between glass- and metal-substrates or a connectionbetween glass- and semiconductor-substrates. For this purpose thestacked one-upon-the-other substrates must be heated to a temperature ofseveral hundred ° C. and a DC voltage of about 1000 Volts must beapplied to them. Electrostatic forces and the migration of ions leadfinally to an irreversible chemical bond at the boundary layer betweenthe individual substrates.

This fabrication method is for instance used in the fabrication ofacceleration sensors or accelerometers. Structures in the range ofmicrometers can be placed in a semiconductor substrate or substratum byanisotropic etching. Herein in many application cases the substrates arenot etched through their entire thickness, so that the substrate remains“non-transparent”. A semiconductor structured in such a way is coveredon both sides by glass substrates for obtaining a functional sensor,wherein the glass substrate also mostly has delicate structures on theside facing the semiconductor substrate, wherein for instance conductorparts and contact strips for the sensor produced by metallizing oretched channels, which on their part must be oriented with respect tothe active structures of the semiconductor substrate participating inthe change of the measured magnitude.

In the fabrication of such multilayer substrate packages a problemoccurs, if one or several of the substrates which have to be alignedwith high precision to one another is or are not transparent.

Methods and devices are known which perform the construction of asubstrata package consisting of several substrates in a layered manner,by to begin with aligning a glass layer with the semiconductor substrateand connect same by bonding and thereupon turn the thus constituted twolayer package, in order to subsequently apply the second glass layer tothe other surface of the semiconductor substrate.

However this method has several disadvantages. On the one hand glass andsemiconductor materials have different coefficients of thermalexpansion, so that in the course of the initial bonding of theintermediate layered packets same bends in accordance with thebimetallic spring principle. The unevenness caused by the arching mustbe compensated by a compressive force during the alignment of the secondglass substrate. In this method it is thus unavoidable, that the thusfabricated multilayer package warps in an unsymmetrical manner.

Devices for performing the described method have often a heatingarrangement, which is designed in such a way, that the substrate packageto be bonded is heated only from one side. During the first bondingprocess the semiconductor substrate is for instance placed upon aheating plate, during the second bonding process the glass substrate isthen placed upon the heating plate. The glass substrate however has aconsiderably lower heat conductivity than the semiconductor substrate.Thus the temperature of the boundary layers to be bonded are in bothcases different. A temperature regulation at the bonding surface doesnot result in any improvement, since the differing temperatures of theheating device again cause warping of the substrate package in adisadvantageous manner.

Another disadvantage in the described known device for performing thefabrication method consists in that in the course of the second bondingprocess the first substrate package is subjected to an electric polereversal, since the heating plate is usually connected solidly to thecathode potential due to the construction of this apparatus. The anodecontacts should however always be applied to the semiconductorsubstrate. An impairment of the mechanical properties of the first bondconnection cannot be completely excluded by the chemical process at theboundary surface triggered by reversal of the poles.

The present invention is now based upon the task of creating a methodand an appropriate device, in order to bond substrates, which haveapproximately the same surface area and of which at least one is nottransparent, with one another while observing a very high adjustment oralignment accuracy to one another in accordance with their structure,wherein cycle times must be achieved, which are suitable for a seriesfabrication of the substrate packages to be bonded.

As a marginal condition the solution must take account, that the devicemust meet all those requirements which apparatus used in a clean roommust satisfy. Freedom from particles is especially one of those, meaningit has to be assured that none of the components of the device producesparticles. Thus for instance no guides causing abrasion wear can be usedin the transportation means and if pneumatic drives are used the wasteair must be directed below the working surface.

The discovered solution solves the problem as defined in an advantageousmanner and completely avoids the disadvantages of the known methods anddevices. The separation in space of the adjustment- and heating-units isparticularly expedient. The adjustment unit, which aligns the positionof the substrate with an accuracy of a few micrometers, is thus notexposed to the high temperatures required for bonding. The length- andvolumetric-expansion of the means for receiving and retaining thesubstrate as well as that of the substrates themselves would, at therequired temperatures, be of the same order of magnitude as theadjustment accuracy required due to the width of the structures or itwould even exceed these. Therefore, in the invention the extremelyaccurate alignment of the substrates to one another corresponding totheir structure is performed in a first working region of a handlingdevice. Thereupon the substrates are then conveyed to another workingregion of the handling device by a transport arrangement and are theredeposited one upon the other in the defined manner, prior to all thesubstrates being fed together to a heating device in the form of asubstrate package containing them in an aligned manner.

All the superimposed substrates are simultaneously bonded in only onework step, wherein the disadvantages of the thermal warping by stepwisebonding are completely eliminated and the fabrication period isconsiderably shortened. The bonding is a fabrication step requiring themost time in the entire fabrication process. This measure decisivelyshortens the cycle time in a fabrication in series, especially if theheating device is laid out so that it can simultaneously accept severalsubstrate packages. Through the simultaneous bonding of all boundarylayers of the substrate package the consequences which arise by polereversal of previously bonded layers are also avoided.

In the following the invention will be described with particularityusing an embodiment example and three Figures.

FIG. 1 shows a substrate package 1 in plan view. In FIG. 2 a cutout Afrom the substrate package in FIG. 1 is shown at a larger scale, whereinadditionally the layered build-up of three substrates 2, 3 and 4 withthe delicate and complex structures applied to them or on them can beseen by means of the folded-back cut. These two Figures clarify theproblem arising if these substrates are to be placed upon one another inthe defined manner and if at least one of the substrates is nottransparent, as in this case for instance substrate 3.

FIG. 3 shows diagrammatically the apparatus or device in the inventionfor performing the process.

The method is subdivided into the following essential work steps:

Adjustment

Transportation

Stacking

Bonding

The adjustment or alignment of a substrate is performed in a firstworking area 10 of a handling device 12, so as not to expose themechanical components of the alignment unit to a high thermal stress.Apart from increased wear and a considerably more expensive constructionof the entire handling device 12, it would be otherwise extremelydifficult at temperatures of 300° C. to observe the required adjustmentaccuracy of less than 10 micrometers in a reproducible manner.

The handling device 12 has two working areas 10 and 11, with means 14for receiving and retaining the substrates provided in each of them;conveyance means 15 connecting the two working areas 10 and 11 to oneanother making it possible to convey one substrate from the first to thesecond working area as well as depositing it there in a defined mannerare also provided.

Receiving means 14 in the first working area 10 consist for instance ofa disk-shaped chuck, upon which a substrate is deposited either manuallyor automatically and retained there by means of negative pressure, assoon as the vacuum suction has been activated. The means for vacuumsuction are not shown in FIG. 3.

The vacuum chuck for receiving the substrate in the first working area10 is supported on an xy-coordinate table 13 rotatable through 360°. Thepositionings are achieved by means of precision drives 16. Thus thedisplacement accuracy for both axes amounts to 1 micrometer.Rotationally an adjustment accuracy of 0.1° is attained.

A depository table 17 is provided in the second working area 11 uponwhich the lower portion of the substrate receptacle rests so as to bearrested or located by two alignment pins 18. The substrate receptacle19 consists of a plate made from an electrically conductive material aswell as being provided on the side of the substrate with channels forsuction by negative pressures and on the rear side with a fixedlyattached magnet 20. A symmetrical matching piece 21 is also depositedguided by the alignment pins upon the substrate receptacle 19 and itholds the interposed substrate package 1 together by means of magneticattraction. The two retaining plates 19 and 21 are connected to oneanother in an electrically conductive manner by means of the twoalignment pins 18. The two glass substrates 2 and 4 resting at theretaining plates 19 and 20 are thus contacted across a large area forthe subsequent bonding process.

In order to avoid thermal warping in the substrate package 1 which is tobe bonded, it is advisable to make the means 19 and 21 for receiving andretaining the substrates out of a material whose coefficient of thermalexpansion corresponds as mush as possible to that of the substrate. Thetemperature coefficients of the substrates to be bonded to one anotherare however as a rule different, however they are mostly of the sameorder of magnitude. It assists greatly in improving the quality of thebonding connection, if the material is selected for the substratereceptacles, whose coefficient of thermal expansion approaches thisorder of magnitude as much as possible, in order to avoid thermalwarping due to heating the entire package.

The conveyance means 15 for conveying the substrate from the first tothe second working area consists for instance of a fork-shapedtransportation sled 23 whose fork is open toward the left hand side, asshown in the sectioned illustration in FIG. 3, so as not to collide withthe microscope system 31, 32. The sled is also equipped with means forreceiving the substrate by vacuum suction, which however are not shownin FIG. 3. The transportation sled 23 is supported with little frictionupon longitudinal guides 24 and can be moved in between fixed stops 22and 25. A hoisting or lifting device 26 is additionally provided inorder to lower the transportation sled 23 down to the adjustment table13 in the first working area 10 or down to the depository table 17 inthe second working area 11. In order to limit the contact force of thesled 23 against the table the hoisting force Z acts against a spring 27.

The problems arising during alignment of the substrate result from thecircumstance, that the semiconductor substrate at least is nottransparent. Therefore the solution in the invention proposes to orientor align each substrate relative to nominal positions. Cross hairs 28,29 in a stationary microscope system 30 are preferably used for thispurpose.

In the preferred embodiment two alignment or test marks of the substrateare imaged by a Splitfield microscope. A Splitfield microscope has theproperty of simultaneously producing more image alignment or test marksseparated in space. In this case it consists of two reflected lightmicroscopes 31 and 32 with coaxial polarized light travel, which areilluminated by a cold light source through optical fibers 33, 34. Thetwo reflected light microscopes 31 and 32 with a lens having a 10-foldmagnification are mounted for instance upon a traverse and their opticparameters can be varied. The image proper is produced by asemiconductor camera respectively on one each monitor 35, 36 as an imagecarrier. Displaceable or adjustable vertical and horizontal measuringlines are faded-in or superimposed upon the image of the substrate bytwo cross hair transmitters. These constitute the stationary nominalpositions, according to which the substrate is aligned in the firstworking area 10. In FIG. 3 the individual components cold light source,semiconductor camera and the control unit required for operating thisapparatus are combined in the field 37.

The alignment marks of the substrate are formed either by the activestructures of the substrate itself or structures applied on or into thesubstrate expressly for the alignment process are used. The actualpositions to be observed on the monitors 35 and 36 are adjusted to thenominal positions predetermined by the cross hairs 28 and 29 by means ofthe precision drives 16 of the coordinate table 13.

In order to be able to meet the required alignment accuracy, a highmechanical stability of the handling device 12 is necessary. Here thespacing between the microscope and the substrate receptacle has to beespecially mentioned, as also the reproducibility of the positioning ofthe transportation sled 23 at the fixed stops 22 and 25.

The pickup and depositing of the substrate must equally be performed inan exactly defined manner, meaning with extreme positional accuracy. Forthis purpose the conveyance sled 23 travels out of its inactive positionover the depositing table 17 in the second working area 11 of thehandling device 12 to the left fixed stop 22 above the alignment table13 in the first working area 10. There it is lowered until its means forreceiving the substrates come to rest upon the aligned substrate whichhas to be transported. A control circuit which is not shown in FIG. 3,monitors the pickup of the substrate by means of vacuum suction by thesubstrate receiving means of the transportation sled 23, since at thesame time the retention of the substrate in the alignment unit must bedisconnected. The s witching point at which a substrate is retainedeither by the transportation sled 23 or by the alignment unit can bevariably adjusted.

After a substrate has been picked up by the transportation sled 23, thesled 23 is raised and travels as far as the righthand fixed stop 25 inthe second working area 11 of the handling device 12, where the sled islowered upon the depository table 17. Only the first substrate which isto be deposited in the second working area is aspirated there by vacuumof the receptacle existing at that point, wherein the transition fromdisconnection in the retaining means of the transportation sled 23 forholding the substrate upon the depository table 17 proceeds in acontrolled manner similar to what occurred in the first working area 10.All additional substrates are only placed upon the previous substrate,wherein, prior to the upward travel of the transportation sled 23 intoits position of rest, an adjustable time delay between the loweringprocess and disconnecting the vacuum is provided in order to eliminatethe air cushion between the substrates.

Once all desired substrates have been stacked one above the other on thedepository table 17 in a manner as is shown in FIG. 3, the top holdingplate 21 guided by the alignment pins 18 is placed upon the substratepackage 1. This upper holding plate 21 thus requires no alignmentmarkers. It is held by magnetic attraction. The entire packageconsisting of stacked substrates and the holding plates 19 and 21embracing the stack is now directed to a heating device 38. Atemperature regulation circuit 39 maintains the adjusted temperaturewithin an accuracy of at least 1° C. It is expedient that the heatingdevice 38 has a temperature indictor 40 and a display instrument 41 forthe bonding voltage supplied from the high voltage source 45 as well asmeans 42 for uniform heating of the substrate package on all sides. Themethod in the invention however does not depend upon these componentsbeing integrated or not into the heating device 38.

The heating device 38 comprises also a high voltage passage 43 into theheating space 44 proper of the heating device 38, in order to supply theelectric energy from a high voltage source 45 required for bonding thesubstrate package 1 located in the heated space 44. Means 46, 47 areprovided in the heating device for electrically contacting the substratepackage, which means connect the anode of the DC voltage source 45through a contact to the semiconductor substrate 3 and connect the glasssubstrate 2, 4 to the cathode across a large area. Multilayeredsubstrate packages can be advantageously bonded simultaneously in thisway in only one work step. It could be advantageous to deposit thesubstrate package on an auxiliary stand 48 in the heating space 44.

The dash-dot bordering 49 of all the units in the device for performingthe method in the invention is meant to indicate that these device unitsare operated in a clean room environment. This applies particularly tothe handling device 12 and the substrates to be fed to same. FIG. 3shows in outline two stacks 50 and 51 of different types of substrates,for instance glass substrates 2 and 4 and semiconductor substrate s 3,which are made available in a magazine or another suitable container notshown here for further treatment at the handling unit 12. After thesubstrates have been stacked one upon another and embraced by the twosubstrate receptacles 19, 21 in a dust-proof manner, the heating device38 can be placed in proximity of the handling device 12 also within theclean space according to purely practical aspects, in order to possiblyenable charging of the heating space 44 by a widening in the handlingdevice 12. This embodiment is expedient, as far as the disposition ofthe heating device 38 is concerned, however contrary to the dispositionof the handling device 12 for performing the inventive method, it is notabsolutely required.

The apparatus created in this manner can completely fulfill therequirements of bonding multilayered substrate packages across largeareas and with small cycling times in a manner suited for seriesfabrication of sensors at uniform heating, wherein the structuredsubstrates have been previously aligned to one another with highaccuracy corresponding to their structuring in a working area separatefrom the heating device. All described measures for the solution in theinvention assist in achieving overall an alignment accuracy of 10micrometers or better in a reproducible manner in the substrate packagesto be bonded after termination of all process steps.

The proposed solution is suitable for automatic fabrication, sincepneumatic or electric drives can easily be used for the mechanicalmovement sequence and even the alignment can, if need be, be performedfully automatically by electronic image evaluation with subsequentpositional regulation of the substrate receptacle in the alignment unit.Equally the apparatus can be built up in a manner suitable for use inclean rooms, wherein all aspects of the task definition are met.

What is claimed is:
 1. A method of stacking a plurality of structuredsubstrates to be aligned with one another corresponding to structuringthereof and connected undetachably to one another by bonding, with thesubstrates having approximately same surface area and with at least onesubstrate being non-transparent, said method comprising the steps of:providing a substrate handling device having spaced first and secondworking areas and substrate receiving means provided at each of thefirst and second working areas; aligning the substrates in the firstworking area of the handling device, said aligning step includingdirecting a microscope upon substrates, arranged on the receiving meansof the first working area, for generating two spatially separatedalignment marks of a substrate on an image carrier, with the alignmentmarks being formed by one of active structures of the substrate itselfand by independent markings, and changing an actual position of thesubstrate by rotating and displacing the receiving means until an imageof the alignment marks coincides with a nominal position present uponthe image carrier; thereafter, transporting the substrates to the secondworking area of the handling device and depositing the substrates oneupon another in accordance with alignment thereof to form a substratepackage; and transporting the substrate package to a heating device foruniform heating of all sides of the package to effect simultaneousbonding of the substrates in a single work step.
 2. An apparatus forstacking a plurality of structured substrates to be aligned with oneanother corresponding to structuring thereof and connected undetachablyto one another by bonding, with the substrates having approximately samesurface area and with at least one substrate being non-transparent, saidapparatus comprising: a substrate handling device having spaced firstand second working areas; first means for receiving and retaining asubstrate in said first working area of the handling means; alignmentmeans located in said first working area for supporting said firstreceiving and retaining means, said alignment means comprising acoordinate table having precision drive means for effecting displacementof said first receiving and retaining means along three mutuallyperpendicular coordinates for changing an actual position of thesubstrate supported in said first receiving and retaining means foraligning the substrate; microscope means provided in said first workingarea for imaging two spatially separated alignment marks on thesubstrate supported in said first receiving and retaining means; secondmeans located in said second working area for receiving and retainingaligned substrates to be deposited one upon another; means fortransporting the substrates aligned in said first working area to saidsecond working area and for depositing the aligned substrates onto saidsecond receiving and retaining means one upon another in accordance withalignment effected in said first working area; and means for uniformlyheating on all sides a substrate package formed in said second workingarea for effecting simultaneous bonding of the substrates of the packagein a single working step.
 3. An apparatus as set forth in claim 2,wherein said microscope means comprises a Splitfield microscope forimaging and depicting the two spatially separated alignment markssimultaneously.
 4. An apparatus as set forth in claim 2, wherein saidsecond receiving and retaining means is formed of a material having acoefficient of thermal expansion which approximates to a coefficient ofthermal expansion of the substrates.
 5. An apparatus as set forth inclaim 2, wherein said heating means includes a temperature regulatingunit having a regulation accuracy of 1° C. or less.
 6. An apparatus asset forth in claim 2, wherein a layout arrangement of said first andsecond receiving and retaining means, said microscope means, saidtransporting means, and said heating means is so selected that analignment accuracy of the aligned substrates is less than 10 microns. 7.An apparatus as set forth in claim 2, wherein a substrate package isformed of multilayered glass-semiconductor glass substrates, whereinsaid heating means is adapted to receive a plurality of the substratepackages formed of multilayered glass-substrates, and wherein saidheating means includes a high voltage source having an anode, which isconnected to the semiconductor substrates, and a cathode, which isconnected to the glass substrates.
 8. An apparatus as set forth in claim7, wherein cathode contacts have a large area when anodic bonding ofglass substrates is effected.